We developed a sturdy thermoplastic (abs) frame. A see through poly vinyl flap with patented edges to not to warp. A floating sill to insure good closure. And a strong molded abs lock-out slide to restrict the pet's movements when necessary. Then we made it 4 sizes for pets up to 120-pound. And with a tamper proof mounting system and easy installation, it's the most economical pet door in the industry